Oaks can have unpredictable breeding cycles, creating a bumper acorn crop one year and conserving energy the next.
Photograph: Graham Turner for the Guardian

One of the great mysteries of the countryside is how in some years groups of oak and beech trees produce huge numbers of seeds and in others almost none. This fluctuation affects the populations of a large variety of animals that rely on the trees’ nutritious nuts for food.

There is a generally accepted theory for why these “mast years” happen. Naturalists believe that by producing, all at once, vast quantities of seeds that are eaten by many creatures, enough survives intact until the spring to form trees.

Reproduction is often aided by birds such as jays, which bury acorns to retrieve later, perhaps forgetting where they stored them. Some acorns are said to pass right through wild boar and are deposited back on the ground complete with fertiliser.

A second notion is that production of quantities of large seeds, such as acorns, could weaken trees. So if you live for several hundred years it is better to conserve and build up your energy for a breeding cycle of three to five years.

Jays, like squirrels, help oaks by hiding acorns, which may then germinate. Photograph: Arco Images GmbH/Alamy

That does not resolve the other question. How do all the beech or oak trees coordinate their mast years to produce vast quantities of seed simultaneously?

It is almost certainly to do with the weather at the time that the trees are flowering. Both species rely on wind-blown pollen reaching female flowers. If all the beech or oaks in a wood flower at once in warm windy conditions then the chances of mass fertilisation must be far greater. A sudden cold snap, like the one this spring, would have the opposite effect.